This invention pertains to a servo system such as that described in the U.S. Pat. No. 3,534,344, Method and Apparatus for Recording and Detecting Information, issued on Oct. 13, 1970. In a system as described in that patent, a movable member is positioned by a servo system acting responsive to signals detected from a prerecorded servo track. The systems have particular application to disc or other types of magnetic memories for use with computers. Usually the servo head is ganged with a plurality of read/write heads such that when the servo head is correctly positioned over a servo track, the data heads are likewise positioned at desired data track locations. For positioning the servo head over a servo track, an error signal is generated responsive to a pair of dibit signals bordering the track. The dibit signals are recorded to each side of the servo track such that the relative magnitude of the dibit signals detected indicates the degree of alignment of the head relative to the track. These dibit signals are supplied to a demodulator for the generation of an error signal responsive to the relative magnitudes of the dibit signals detected from the servo track.
In the usual manner the demodulator functions to generate a dc output signal responsive to the peak values of the dibit signals at the side of the servo track. Such signal is caused to decay between each dibit pulse and be reset with each subsequent pulse at a value equal to the amplitude of the dibit pulse. It is necessary for the signal to decay between each pulse so as to accommodate the circumstance of the pulses becoming smaller in magnitude as the servo head moves away from that side of the servo track. Naturally the faster the servo head is being moved relative to the track, the more it is necessary for the output signal to decay so as to reach a level below the next dibit pulse magnitude and thereafter be reset higher for an accurate indication of that signal magnitude. Thus for the generation of an accurate output signal in instances of rapid change of the signal, it is necessary that the output pulse decay rapidly between dibit pulses.
However such prerecorded servo signals while having the maximum magnitude closely controlled are subject to noise commonly referred to as dropout. Such noise results from the absence of the dibit signals usually due to a flaw or discrepancy in the media preventing the servo signal from being recorded. Thus it is also necessary for the servo system to function in a manner to accommodate periodic missing dibits which result from discrepancies in the media. One method of such accommodation is to cause the output signal to decay at a slow rate between dibits so as to maintain the resulting error signal at or near the value of the most recent detected dibit which is the most accurate indication of the present position of the servo head in the absence of the subsequent dibit signal. Thus if the dibit signal is absent, the servo system will exhibit an inertial or flywheel effect to more or less maintain the servo head at the position dictated by the last servo track information supplied by the servo transducer until a dibit signal is again detected.
Thus as can be seen, the first requirement that the output signal decay rapidly is counter productive to the need for the servo system to accommodate noise or dropout in systems used up to now. In other words the accommodation of either one of the before-described conditions can only be achieved at the expense of the other requirement in prior systems. It is the purpose of this invention to provide a system which will accommodate both requirements, namely an output signal that will follow a quick decaying dibit signal while yet accommodating noise or dropout in the servo signal.